A diaphragm carburetor has been employed for a small general-purpose internal combustion engine which should be operated in a variety of attitudes.
As is well known, at the initial start of the engine, a richer mixture than that used in a normal operation is supplied thereto for facilitating the starting. Because of this, in general purpose engines, a choke valve is first fully closed and then starting operation is carried out. However, concentration of a mixture required at the time of starting considerably varies depending on the temperature of the engine. Therefore, even if the concentration of the mixture is optimum at the low temperature, the mixture is sometimes excessively rich at the normal temperature.
Furthermore, in restarting the engine after the engine has already been operated, a mixture of normal concentration will suffice to be supplied since the temperature of the engine is higher than that of the initial starting. However, in restarting the engine when a quantity of fuel in a fuel tank is insufficient, fuel in the carburetor is insufficient or absent. Therefore, the choke valve is fully closed after the fuel has been supplied to the fuel tank, and then starting operation is carried out. In this case, however, the carburetor is filled with fuel and at the same time, a super-rich mixture is supplied to the engine. When the super-rich mixture is supplied to the engine, roping (starting operation, that is, pulling the recoil rope) has to be effected over and over again, especially when a throttle valve is in an idling position. It is usually necessary to move the choke valve to a fully open position to completely discharge the super-rich mixture. Furthermore, an ignition plug may be covered with the mixture depending on the displacement of the engine and the arrangement of the ignition plug, thus rendering starting impossible.
It is therefore an object of the present invention to provide a choke mechanism for a carburetor having an excellent startability which overcomes various starting conditions of the engine as described above.
For achieving the aforesaid object, the present invention provides an arrangement wherein a choke valve is formed with a leak hole, and a valve plate formed of temperature responsive alloy is superposed on the choke valve, said valve plate closing the leak hole at the time of low temperature while opening the leak hole at the time of high temperature.